The invention refers to a device for guiding moving articles past at least one ink-jet printing head,.
As compared to mechanical printers, the use of ink-jet printers for canceling stamps has the advantage that the stamp layout can be designed freely and that it is possible to achieve a high printing quality. The layout can vary from article to article. The wear and tear is lower for these systems. In addition, advertising logos can be printed on in this way, which change from article to article.
The articles must be guided precisely while moving past the ink-jet printers. This is achieved with the aid of guide frames. Prevailing guide frames (see FIG. 3) consist of a level surface, along which the articles 10 are guided. This guidance is intended to ensure the necessary distance between article 10 and ink-jet printing head 3. The guide frame is provided with cutouts for the ink-jet printing heads 3, in which the printing heads are positioned.
Guiding the articles with these guide frames has the following disadvantages: The ink used for the printing is an oil-based ink. This ink does not dry through evaporation of a chemical substance, but is soaked up by the material used for the article. The absorption of the ink depends on the material to be printed on. If the surface is very smooth, less ink is absorbed than with a rough surface. This can even lead to very fine ink droplets spraying back, which then settle in the region surrounding the printer. The speed of these drops is clearly lower than the speed of the drops coming directly from the printing head (approximately v=11 m/s). The drops with the lower mass and the low speed can be easily influenced by an air flow.
Owing to its inherent speed, each article generates an air flow directly above its surface. Turbulences form as a result of the sharp-edged transition from the guide frame to the printing head. These turbulences cause the fine drops spraying back to be deposited on the jet backing plate for the printing head. With a continued printing operation, the fine ink drops combine to form large-surface drops, which can clog the jets of the printing head.
In order to prevent these deposits and as shown in FIG. 4, it is known to direct a forced flow 8 onto the jet backing plate. This causes a "blowing clean" of the jet backing plate or prevents a settling down of fine drops in the jet region. The air flow is guided through fine flow ducts from the back toward the jet backing plate (see FIG. 4). The disadvantage here is that a blower must additionally be installed to generate the flow. Owing to the short distances, a considerable design and production-technology expenditure is necessary to direct the flow toward the guide frame.
Another problem is created by articles coming in contact with the printing heads. The articles are not stiff or flat, as would be ideal. Owing to the short distance between article and printing head, the articles frequently come in contact with the jet backing plate, which leads to an increased amount of ink released by the jets. Owing to the high throughput, it is not possible to supply enough ink to the jets and air enters the jets. This leads to a decrease in the printing intensity up to the complete failure of the jets.
It is therefore the object of the invention, to create a device for guiding moving articles past ink-jet printers, which device prevents, with low expenditure a rapid clogging of the printing jets as a result of deposited ink droplets.